1. Field of the Invention
This invention is in the field of avian coccidiosis and relates to effective live oocyst vaccines for coccidiosis and their development through incorporation of new immunovariant derived strains of Eimeria maxima. 
2. Description of the Relevant Art
Coccidiosis is an intestinal disease caused by avian protozoan parasites and is estimated to cost the U.S. poultry industry over $400 million annually. The major component in coccidiosis control in the poultry industry since the 1950s has been the use of anticoccidial compounds. These compounds, when used in carefully designed prophylactic treatment programs, are efficient in disease control. However, all anticoccidial compounds now cleared for use in chickens are showing diminished efficacy due to increased parasite drug resistance. No new anticoccidials are currently being cleared for use. This has resulted in a re-examination of another type of coccidial control, the use of live oocyst-based vaccines.
Vaccination against coccidiosis is not a new concept; it has been used by the poultry industry since the early 1950s. The four dominant commercial vaccines used by the poultry industry, COCCIVAC®, IMMUCOX®, Livacox, and PARACOX®, typically incorporate several species and strains of Eimeria. Some of the species in the vaccines are attenuated, egg-adapted and/or precocious lines, while others are strains that were originally isolated from commercial poultry production facilities. While all of these vaccines provide solid immunity to coccidial infection when applied carefully under good rearing conditions, especially in replacement and breeding flocks, these vaccines have not been universally accepted by the U.S. poultry industry for meat-producing broiler and heavy roaster bird flocks. The two major reasons for this reluctance has been (1) that bird performance, as measured by weight gain and feed efficiency, has not always equaled that seen with prophylactically medicated birds and (2) that reports of immunological variability of E. maxima, one of the major pathogenic and economically important coccidial species, indicate that vaccination with a given suspension of live oocysts may not be effective in protecting against field strains In differing geographical locations (Long et at 1986. Avian Pathol. 15: 217–278; Long et al., 1979. Parasit. 79: 451–457; Rose, M. E. 1982. In: The Biology of the Coccidia, P. L. Long, Ed., University Park Press, Baltimore Md., pages 329–371; Fit-Coy, S. H. 1992. Avian Dis. 36: 40–43; Lee, E.-H. 1993. In; Proceedings of the Vith International Coccidiosis Conference, Barta and Fernando, Eds., University of Guelph, Guelph, Ont., Canada, pages 118–121).
Danforth et al. described the effect of immunization with oocysts on weight gain and feed conversion in two separate battery trials (Danforth et al. 1997. Avian Dis. 41: 792–801). Three different strains of chickens of either sex were immunized by oral gavage at 1 day of age. Each bird was immunized with 2500 oocysts of a virulent field strain isolate of Eimeria maxima and challenged with 25,000 oocysts of the same strain at 10 days of age. All immunized birds of all strains had significantly higher average weight gains and lower lesion scores at 7 days post-challenge (PC) than that seen in the non-immunized, challenged groups. However, feed conversions were lower for the the immunized groups than the non-immunized birds. All immunized birds, except for strain females, had significantly higher average weight gains than the non-immunized, challenged birds for the length of the experiment (1–17 days of age). None of the immunized birds had significantly higher average weight gains than the non-immunized, unchallenged groups for the entire experiment. These results are in general accord with studies reported previously which indicated that immunization of 1-day-old broiler chicks with oocysts would elicit partial protection against challenge by the same species (Bafundo, K. W. 1989. INRA, Tours, 395–400; Long et al. 1986, supra).
The immunization of 2500 oocysts/bird did however depress weight gains of the birds in both battery trials at 10 days of age. Similar observations were seen for average weight gains (1–17 days of age) of immunized birds irrespective of challenge. The results of preliminary work carried out in caged battery trials indicated that an immunizing dose of 2500 oocysts/bird was necessary to elicit consistent and significant protection against challenge; doses of 500 or 1000 oocysts did not always reduce the severity of the mid-intestinal lesions, or protect against weight loss. Seemingly, the loss in productivity caused by the high immunization dose could not be made up within the first 17 days of age.
At the present time, less than 7% of all broiler meat birds produced in the U.S. (approximately 500,000,000 out of over 7.5 billion birds processed annually) use live oocyst vaccines. This is because of limited effectiveness against field strains of coccidia present in commercial grow-out facilities. For example, the strain of E. maxima present in one of the anti-coccidial vaccines, Immucox® as an example, does not always elicit sufficient immunity when challenged with heterologous field strains of this species (Lee, supra). Immucox® vaccine primarily incorporates strains from Canada. Recent work has shown considerable immunovariation in five strains of E. maxima taken from different geographical areas of North America (Martin et al. 1997. Int. J. Parasitol. 5: 527–533). Immunization with the Guelph laboratory strain E. maxima-GLP, a Canadian isolate, was 97% protective for homologous challenge, but had only 3% and 46% effectiveness in reduction of lesion scores after challenge with a U.S. isolate from Maryland, i.e., the USDA laboratory strain 68, and strain FL, a U.S. isolate from Florida, respectively. Immunization with strain 68 or FL protected against homologous challenge only (65% and 80%, respectively). Immunization with strain MD (another strain from Maryland) and NC (from North Carolina) protect against both homologous (88% and 69%, respectively) and heterologous challenge with the E. maxima-GLP, i.e., 85% and 74%.
These results suggest that Immucox® may not afford protection against challenge with certain strains of E. maxima present in the field (Danforth et al. 1997. Parasitol. Res. 83: 445–451; Martin et al., supra). This was confirmed when caged battery broiler birds were immunized by gavage with Immucox® and challenged with E. maxima strain MD; there was no protection with regard to either weight gain or lesion scores (Danforth et al. 1997, supra). However, if E. maxima strain MD was included in the Immucox® vaccine, then lesion scores resulting from challenge were reduced and weight gain was increased during challenge and throughout the entire experiment (Danforth, H. D. 1998. Int. J. Parasitol. 28: 1099–1109).
Based on these caged battery results, floorpen cage studies were used to measure the performance (as measured by average weight gain and feed conversion) of male roaster birds vaccinated by gel-delivery (IG12) with a reformulated Immucox® vaccine (containing the four original vaccine species, E. acervulina, E. maxima, E. necatrix and E. tenella, and oocysts of strain MD E. maxima) against absolute non-immunized-unchallenged (NINC), gavage-immunized with the reformulated vaccine (IGa2) and nicarbazin-narasin/narasin (Maxiban® and Monteban®, Elanco Products Co.) medicated (NIN/N) control groups. All birds were exposed to wood-shaving litter in the floorpens seeded with 2000 oocysts of strain MD E. maxima/g of litter. The gel-immunized birds performed as well as the anticoccidial-medicated non-immunized groups at the end of a 70 day growout period (Danforth 1998, supra).
In subsequent field trials, the same strain of male roaster birds immunized by gel-delivery at the hatchery with the reformulated Immucox® vaccine, performed similarly to non-immunized birds in control houses that had been medicated with the anticoccidial compound salinomycin (Sacox®, Hoechst Roussel Agri-Vet Co.). The immunized birds were again lighter at 4–5 weeks of age than the control birds, but were heavier by the time of processing (9–10 weeks of age). At the time of processing, adjusted feed conversions for the immunized birds averaged 3.75 points lower than those of the control, medicated birds. (Feed conversion was adjusted to an 8 lb bird using 5 points weight, i.e., 0.01 lb above 8 lb=1 pt.) In addition, total mortality in the immunized birds was lower for the entire growout than that seen in the controls. These results showed that a reformulated Immucox® viable oocyst vaccine, gel-delivered to 1 Day old birds, could control coccidiosis in commercial poultry operations and result in performance that was equal to, or better than, the anticoccidial-medicated birds.
Clearly, while available methods for the control of coccidiosis have met with success, the need for a vaccine that is capable of combating avian coccidiosis by effectively protecting against all field strains of Eimeria maxima remains. Improvement of live oocyst vaccine efficacy by the addition of immunovariant strains of coccidia would give better effective control of this economically important parasitic disease.